1. Field of the Invention
The present invention relates to piezoelectric microblowers used to transport a compressible fluid, such as air.
2. Description of the Related Art
The generation of heat inside compact electronic devices, such as notebook computers and digital AV devices, for example, is a significant problem. It is important and necessary that cooling blowers used in such devices are compact, have a low profile, and have low power consumption.
Driving units that are used in cooling blowers include a diaphragm that is bent and deformed by a piezoelectric member. Generally, a vibrating plate is provided as a diaphragm that is made of a thin resin or metal plate to which a piezoelectric element is attached. Advantageously, this structure has a low profile and low power consumption. Airflow is generated by applying an alternating voltage to the piezoelectric element so as to cause bending deformation, whereby the pressure in a blower chamber is changed. In this kind of piezoelectric microblower, there has been a problem in that if the size of the vibrating plate is reduced so as to reduce the size of the blower, the displacement is significantly reduced, whereby the flow rate is reduced and the desired cooling effect cannot be obtained. Therefore, it has not been possible to sufficiently reduce the size of such blowers.
In Japanese Unexamined Patent Application Publication No. 2008-14148, a jet-flow-generating apparatus is disclosed that includes a casing, a vibrating actuator, and a nozzle member. The vibrating actuator includes a magnet, a vibrating plate on which a driving coil is mounted, an elastic support member that supports the vibrating plate, and a yoke. Where the characteristic frequency of the vibrating plate inside the casing satisfies the conditions for Helmholtz resonance in the casing, the noise is increased. Therefore, the characteristic frequency of the vibrating plate is set so as to be different from the Helmholtz resonant frequency of the casing. Specifically, for a Helmholtz resonant frequency of the casing of 1.09 kHz and a characteristic frequency of the vibrating plate of around 1 kHz, the material of the vibrating plate is changed or a rim or a portion at which the thickness partially changes is provided in the vibrating plate to change the rigidity of the vibrating plate, whereby the characteristic frequency of the vibrating plate is changed to 1.4 kHz to 2.4 kHz. However, if the resonant frequency of the casing is 1.09 kHz and the cavity volume is 1.5×10−5 m3, for example, the casing has approximate dimensions of 100 mm×30 mm×5 mm and cannot be used for very compact mobile appliances. Furthermore, at a driving frequency of 1 kHz, since it is within the audible range, noise becomes a problem.
In Japanese Unexamined Patent Application Publication No. 2008-14148, in order to reduce noise, the resonant frequency of the air inside the blower chamber is set to be different from the resonant frequency of the vibrating plate, because the resonant frequency is within the audible range. When the vibrating plate is driven at a frequency outside of the audible range, noise is no longer a problem.
Accordingly, in a gas-flow generator described in Japanese Unexamined Patent Application Publication No. 2006-522896, an ultrasonic driver is provided which includes a stainless steel disk having a larger diameter than a piezoelectric material disk that is sandwiched between the piezoelectric material disk and a diaphragm (stainless steel membrane) (see FIG. 1 and paragraph 0018 of Japanese Unexamined Patent Application Publication No. 2006-522896). Since ultrasonic driving is performed in a region outside of the audible range by using the third-order resonance mode of piezoelectric bending vibration, the problem of noise does not arise. Driving performed in the first-order resonance mode is preferable since the maximum displacement is obtained. However, first-order resonant frequencies may be within the audible range and, thus, noise is a problem. In contrast, in the third-order resonance mode, the amount of displacement is smaller but, since the frequency is outside of the audible range, noise is not a problem. However, if the diameter of the diaphragm is reduced to attempt to reduce the size of the blower, since the displacement is significantly reduced, the characteristics of the blower are deteriorated and the desired cooling effect is not obtained.